1. Field of the Invention
The present invention relates to systems and methods for constructing cutoff walls. In particular, the Present invention relates to a system and method for producing a cutoff wall with improved impermeability by inserting a lining into a slurry wall.
2. Description of Related Art
Cutoff walls are typically used to impede the lateral flow of groundwater. Controlling the flow of ground water is essential to preventing contamination of the water supply. Cutoff walls are often used to isolate contaminated landfills and to prevent the ground water from being contaminated through contact with buried waste. Cutoff walls have also been used to seal dams, canal systems, and dikes for flood control purposes because of their low permeability. Other applications for cutoff walls include: dewatering and protecting deep excavation projects, hydraulically isolating lagoons and holding ponds, and enclosing oil and chemical tank farms.
As illustrated in FIG. 1, slurry cutoff walls are generally constructed by excavating a narrow trench (2-4 feet wide) with a backhoe or similar device. During the excavation process, the trench is filled with slurry and maintained at a level near the top of the trench. The use of slurry, conventionally of bentonite and water, allows excavation without the need for other lateral support. The narrow trench is then backfilled with excavated soil and selected impervious materials to create a cutoff wall. The selected materials are typically commercially available clays and cement. One advantage of the slurry wall technique is that it is relatively inexpensive to construct a cutoff wall.
However, one problem with the cutoff walls of the prior art is their permeability. Slurry cutoff walls generally have a permeability that reaches at best 1.times.10.sup.-6 to 5.times.10.sup.-7 cm/sec. This becomes a problem when very low permeability is required such as when isolating hazardous wastes as shown in FIG. 2. When the waste site is near an aquifer or other ground water source the permeability must be at least 1.times.10.sup.-8 cm/sec. Very low permeability cutoff walls are also needed for flood control purposes to prevent seepage through dikes, influence on the areas surrounding the cutoff wall and the collapse of dikes. Therefore, there continues to be a need for a system and method for efficiently producing cutoff walls with very low permeability.